Wavelength division multiplexing optical module

ABSTRACT

Examples herein relate to optical modules. In particular, implementations herein relate to optical modules that include top-emitting VCSELs and/or top-entry photodetectors. The optical modules include a first interposer having opposing first and second sides and a second interposer having opposing first and second sides. The optical modules include a plurality of top-emitting vertical-cavity surface-emitting lasers (VCSELs) coupled to the second interposer and a plurality of electrical conductors forming electrical paths between electrical contacts of the top-emitting VCSELs and the second side of the second interposer. The VCSELs are configured to emit optical signals having different wavelengths. The optical signals are configured to be combined and transmitted over a single optical fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to commonly assigned U.S. application Ser.No. 16/397,083, filed on Apr. 29, 2019, the contents of which is herebyincorporated by reference in its entirety.

BACKGROUND

Optoelectronic communication (e.g., using optical signals to transmitelectronic data) is becoming more prevalent as a potential solution, atleast in part, to the ever increasing demand for high bandwidth, highquality, and low power consumption data transfer in applications such ashigh performance computing systems, large capacity data storage servers,and network devices. Wavelength division multiplexing (WDM) is usefulfor increasing communication bandwidth by combining and sending multipledata channels or wavelengths from multiple optical sources over anoptical fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain examples are described in the following detailed description andin reference to the drawings, in which:

FIG. 1A schematically illustrates a section view of a block diagram ofan example of an optical module according to the present disclosure;

FIG. 1B schematically illustrates a section view of a block diagram ofanother example of an optical module according to the presentdisclosure;

FIG. 1C schematically illustrates a section view of a block diagram ofanother example of an optical module according to the presentdisclosure;

FIG. 2A schematically illustrates a section view of a block diagram ofanother example of an optical module according to the presentdisclosure;

FIG. 2B schematically illustrates a section view of a block diagram ofanother example of an optical module according to the presentdisclosure;

FIG. 2C schematically illustrates a section view of a block diagram ofanother example of an optical module according to the presentdisclosure;

FIG. 3A schematically illustrates a top section view of a block diagramof an example die layout of an array of optical modules according to thepresent disclosure;

FIG. 3B schematically illustrates a top section view of a block diagramof another example die layout of an array of optical modules accordingto the present disclosure; and

FIG. 4 schematically illustrates a section view of a block diagram ofanother example of an optical module according to the presentdisclosure.

DETAILED DESCRIPTION OF SPECIFIC EXAMPLES

The present disclosure describes various examples of a WDM opticalmodule that includes a plurality of laser sources to emit and/or aplurality of photodetectors to receive optical signals having differentwavelengths. For example, the laser sources can be top-emittingvertical-cavity surface-emitting lasers (VCSELs) and the photodetectorscan be top-entry photodetectors coupled to a second interposer. Amultiplexer can be used to join the optical signals emitted by theVCSELs through a first interposer together before transmitting them overan optical fiber. The first interposer coupled to and disposed above thesecond interposer. A demultiplexer of an optical receiving modulecoupled to an opposing end of the optical fiber can subsequently be usedto separate the optical signals transmitted by the optical fiber to bereceived by respective photodetectors, as described in more detailbelow.

The first interposer can be constructed of glass or other suitablematerials with a relatively high-index of refraction (e.g., GaAs, GaP,GaN, InP). For example, the first interposer can be a glass die orwafer. The first interposer is disposed over or above the secondinterposer such that the top-emitting VCSELs or the top-entryphotodetectors are disposed between the interposer and the substrate. Asdescribed herein, the “second interposer” can refer to an organicbuild-up substrate, a Si-interposer, an SOI interposer, integratedcircuit (e.g., ASIC), chip, die, or printed circuit board depending onthe application. Further, the optical module includes electricalconductors forming electrical paths between top-side electrical contactsof the top-emitting VCSELs (or the top-entry photodetectors) and thesecond interposer. The second interposer can further includeelectrically conductive traces or vias to pass electrical signals to orfrom the electrical conductors to an integrated circuit (e.g., an ASIC,driver integrated circuit, receiver integrated circuit) for driving thetop-emitting VCSELs or processing electrical signals converted by thetop-entry photodetectors.

An “optical fiber” as described herein can refer to a single opticalfiber (e.g., including a core and a cladding) to provide unidirectionalor bidirectional optical communication, can refer to a bidirectionalpair of optical fibers (e.g., each including a core and a cladding) toprovide both transmit and receive communications in an optical network,or can refer to a multi-core fiber, such that a single cladding couldencapsulate a plurality of single-mode cores.

FIGS. 1A-1C illustrate examples of optical modules 100 (identifiedindividually as optical modules 100 a-100 c) and components thereofaccording to the present disclosure. Each of the optical modules 100a-100 c can include one or more of any of the components or features, inwhole or in part, of any of the features described herein with respectto each other. An optical module as described herein can include or formpart of an optical transmitter (e.g., including top-emitting VCSELs), anoptical receiver (e.g., including top-entry photodetectors), or both anoptical transmitter and receiver. The optical module 100 includes asubstrate 102 having opposing first and second sides (e.g., top andbottom sides). As described above, the substrate 102 can be an organicbuildup substrate. The optical module 100 includes an interposer 104having opposing first and second sides disposed over or above thesubstrate 102. The interposer 104 can be formed out of glass or othersuitable material(s) with a relatively high-index of refraction. Theinterposer 104 can include a plurality of lenses 106 (identifiedindividually as lenses 106 a-106 d) integrated on or otherwise formed onthe first side of the interposer 104. For example, the lenses 106 may befabricated via spin coating a polymer or other film on the interposer104. While illustrated as protruding out in a convex manner, in otherimplementations, the lenses 106 can be recessed within the interposer104.

The optical module 100 includes a plurality of top-emittingvertical-cavity surface-emitting lasers (e.g., VCSELs 108) flip-chippedto the second side of the interposer 104. As illustrated, thetop-emitting VCSELs 108 are disposed between the substrate 102 and theinterposer 104 and are configured to emit optical signals havingdifferent respective channels or wavelengths (e.g., λ₁, λ₂, λ₃, λ₄). Theoptical signals are configured to be combined and transmitted over asingle optical fiber 110 (e.g., via a multiplexer 112), as described inmore detail below. While illustrated as having four top-emitting VCSELs108 (identified individually as VCSELs 108 a, 108 b, 108 c, and 108 d),the optical module 100 can include more than four top-emitting VCSELs(e.g., eight, sixteen, thirty-two, sixty-four) and emit additionalwavelengths to be combined respectively (e.g., to increase overallbandwidth accordingly).

The optical modules described herein (e.g., optical modules 100 a-100 c,200 a-200 c, and 400) can be optical transmitters, receivers, ortransceivers. For example, the optical modules can include a pluralityof top-emitting VCSELs, top-entry photodetectors, or both. With the useof top-emitting VCSELs or top-entry photodetectors rather thanbottom-emitting VCSELs or substrate-entry photodetectors and theelectrical and optical input/output (IO) configurations describedherein, the optical modules can support wavelengths in both the onemicron range (e.g., 980 nm to 1100 nm) for coarse wavelength divisionmultiplexing (CWDM) as well as in the 850 nm range (e.g., 840 to 940 nm)for short wavelength division multiplexing (SWDM). Further, astop-emitting VCSELs or top-entry photodetectors are provided with theoptical modules described herein, the top-emitting VCSELs or top-entryphotodetectors can be fabricated with either transparent ornon-transparent support substrate layers.

Further, as described above, the top-emitting VCSELs can be replaced orsubstituted with top-entry photodetectors configured to receive opticalsignals from the optical fiber and convert the optical signals toelectrical signals for further processing. In the interest of clarityand to avoid unnecessarily obscuring the description, many of thevarious examples described herein refer to optical modules withtop-emitting VCSELs. However, any of the example optical modulesdescribed herein can include top-entry photodetectors in addition to thetop-emitting VCSELs (e.g., to form optical transceivers with transmitterand receivers) or in place of the top-emitting VCSELs (e.g., to form anoptical receiver module).

The optical module 100 includes a plurality of electrical conductors 114(e.g., identified individually as electrical conductors 114 a-114 d).Each of the electrical conductors 114 forms a respective electrical pathbetween electrical contacts 116 (e.g., identified individually aselectrical contacts 116 a-116 d) of respective top-emitting VCSELs 108and the substrate 102. The electrical contacts 116 of each top-emittingVCSEL 108 include a pair of contacts (e.g., an anode contact and acathode contact) disposed on a top side of the top-emitting VCSEL suchthat respective electrical paths extend or are routed from the top sideof the top-emitting VCSEL 108 downward to the substrate 102. Asdescribed herein, the top side of the top-emitting VCSEL 108 refers toany VCSEL layers above a VCSEL support substrate of the top-emittingVCSEL 108. Typically, top-emitting VCSELs include one or more activelayers sandwiched between upper and lower mirror layers built orotherwise formed on the support substrate. Therefore, by being disposedon the top side of the top-emitting VCSEL, the electrical contacts 116are disposed above or over the support substrate.

As illustrated, only one of the pair of electrical contacts 116 (e.g.,the cathode or anode contact) of each top-emitting VCSEL is shown orvisible. In some examples and die layouts, the other non-visibleelectrical contact 116 of the pair of contacts in the section viewsherein (FIGS. 1A-2C) is disposed behind or in the same row or column asthe visible electrical contact on a same lateral side of a mesa of thetop-emitting VCSEL 108 (see FIG. 3A). In other examples or die layouts,the pair of electrical contacts 116 are disposed on opposite lateralsides of the top-emitting VCSEL 108 or mesa (see FIG. 3B).

With reference to FIG. 1A, in some examples, each of the electricalconductors 114 of optical module 100 a can include electricallyconductive metal pillars 118 (identified individually as metal pillars118 a-118 d) such as solid metal pillars or metal lined pillars. Themetal pillars 118 can be copper pillars or constructed out of othersuitable metal material. The metal pillars 118 extend between andelectrically couple respective first pads 120 (identified individuallyas first pads 120 a-120 d) disposed on the second side of the interposer104 and respective second pads 122 (identified individually as secondpads 122 a-122 d) disposed on the first side of the substrate 102. Themetal pillars 118 can be fabricated or otherwise formed on theinterposer 104. The metal pillars have heights (e.g., 175 to 225micrometers) that are greater than heights of the top-emitting VCSELs108 (e.g., 150 to 200 micrometers) to provide sufficient clearance forthe top-emitting VCSELs 108 between the interposer 104 and substrate102.

The pads as described herein (e.g., the first and second pads 120 and122) can be, for example, solder attachment pads. Each of the first pads120 are electrically coupled to one of the electrical contacts 116(e.g., one of the anode or cathode contacts) of a respectivetop-emitting VCSEL 108 (e.g., via corresponding or matching pads ortraces of the top-emitting VCSEL 108). The electrical metal pillars 118thus form electrical paths between the respective electrical contacts116 on top sides of the top-emitting VCSELs 108 and the substrate 102thereunder. As discussed above, the optical module 100 includes otherelectrical contacts 116 (e.g., the other of the anode or cathodecontacts) of the pair of electrical contacts (not shown in FIGS. 1A-1Cor 2A-2C). These electrical contacts 116 are also electrically coupledto the substrate 102 via separate electrical conductors 114 extendingbetween the substrate 102 and interposer 104.

The first pads 120 can be single pads (e.g., continuous pads) extendingalong the interposer 104 and attached to both the electrical conductors114 and the top-emitting VCSELs 108. However, in other examples, thefirst pads 120 can include separate pads (e.g., two pads) configured tobe coupled to the electrical conductors 114 and top-emitting VCSELS 108respectively. The separate pads can be electrically coupled viaconductive traces extending along or through the interposer 104.

The substrate 102 can also include conductive traces 128. The secondpads 122 can be electrically coupled to an ASIC 130 or other suitablechip via the traces 128 (identified individually as traces 128 a-128 d)such that the ASIC 130 or other suitable chip can send electricalsignals to respective top-emitting VCSELs 108 via the electrical pathsbetween the substrate 102 and interposer 104 formed by the electricalconductors 114, pads (e.g., first and second pads 120 and 122, traces,and contacts 116. When the optical module 100 includes top-entryphotodetectors, electrical signals converted from optical signals by thetop-entry photodetectors can be sent to the ASIC 130 or other suitablechip via the electrical paths between the substrate 102 and interposer104 for further processing.

As illustrated, the top-emitting VCSELs 108 (or top-entryphotodetectors) can be mechanically coupled to respective first pads 120via soldering (e.g., solder bumps 126 and corresponding solder reflowtechniques). The optical module 100 can also include additional pads 124(identified individually as pads 124 a-124 d) mechanically coupled tothe respective top-emitting VCSELs 108 via soldering. Further, the metalpillars 118 can be mechanically coupled to respective second pads 120via soldering as well. During a solder reflow process, the top-emittingVCSELs can be passively aligned with the respective lenses 106 (e.g.,due to precise arrangement or position of the pads 120 and 124 on theinterposer 104).

In some examples, an optical underfill layer 132 can be provided on thesecond side of the interposer 104 to reduce or prevent opticalreflections. The optical underfill layer 132 is optically transparentand has an index of refraction matching or substantially matching thatof the interposer 104. In some examples, the optical module 100 caninclude a thermally conductive underfill layer 134 under the opticalunderfill layer 132 to improve heat flow from the top-emitting VCSELs108 to the substrate 102. Further, the first side of the substrate 102can include heat spreaders 136 (identified individually as heat sinks136 a-136 d) or heat sinks for distributing the heat from respectiveVCSELs 108.

As discussed above, the optical module 100 can include the multiplexer112 and the optical fiber 110 or other suitable waveguide. Opticalsignals of varying wavelengths emitted by the top-emitting VCSELs 108can be collimated by the respective lenses 106 and multiplexed orotherwise combined by the multiplexer 112 (e.g., a zig-zag multiplexerwith a plurality of filters and reflectors). The combined opticalsignals can then be transmitted by the optical fiber 110 to, forexample, another optical module, chip, or device. The optical module 100can include an optical connector assembly (e.g., ferrule and socket) tocouple the optical fiber 110 and multiplexer 112 to the interposer 104or substrate 102. When the optical module 100 includes top-entryphotodetectors, the multiplexer 112 can demultiplex optical signalstransmitted to the optical module 100 via the optical fiber 110 to bereceived by the top-entry photodetectors. For example, opposing ends ofthe optical fiber 110 can be coupled to optical modules (e.g., opticaltransmitter and receiver modules, including top-emitting VCSELs andtop-entry photodetectors, respectively).

With reference to FIG. 1B, in some examples, the optical module 100 bincludes a plurality of standoffs 140 (identified individually asstandoffs 140 a-140 d) or posts disposed on the second side of theinterposer 104. The standoffs 140 can be, for example, silicon (Si)standoffs bonded to or otherwise fabricated on the second side of theinterposer 104. The standoffs 140 have heights (e.g., 175 to 400micrometers) that are greater than heights of the top-emitting VCSELs108 (e.g., 150 to 200 micrometers) to provide sufficient clearance forthe top-emitting VCSELs 108 between the interposer 104 and substrate102. The standoffs 140 extend between the interposer 104 and thesubstrate 102. While illustrated as having angled or slanted sidewalls,in other examples, the standoffs 140 can have vertical or substantiallyvertical sidewalls.

Each of the electrical conductors 114 includes conductive traces 142(identified individually as conductive traces 142 a-142 d) extendingalong and supported by a respective standoff 140. The conductive traces142 extend between and electrically couple respective first pads 120(identified individually as first pads 120 a-120 d) disposed on thesecond side of the interposer 104 and respective second pads 122(identified individually as second pads 122 a-122 d) disposed on thefirst side of the substrate 102. As described above, the first pads 120are electrically coupled to one of respective electrical contacts 116 ofthe top-emitting VCSELs 108. The conductive traces 142 thus formelectrical paths between the respective electrical contacts 116 on topsides of the top-emitting VCSELs 108 and the substrate 102 thereunder.

With reference to FIG. 1C, in some examples, the optical module 100 cincludes both the plurality of standoffs 140 disposed on the second sideof the interposer 104 extending toward the substrate 102 and the metalpillars 118 as described above with respect to FIGS. 1A-1B. Each of themetal pillars 118 extends from a respective standoff 140 downward to thesubstrate 102. A combined height of the metal pillars 118 and standoffs140 from which the metal pillars 118 extend is greater than a height ofthe top-emitting VCSELs 108 to provide sufficient clearance for thetop-emitting VCSELs 108 between the interposer 104 and substrate 102.The metal pillars 118 can be shorter in height (e.g., 50 to 100micrometers) relative to those illustrated in FIG. 1A as the standoffs140 provide additional clearance height. In some examples, shorter metalpillars 118 may be easier to fabricate.

Each of the electrical conductors 114 includes a first portion (e.g.,conductive traces 142) extending along respective standoffs 140 and asecond portion (e.g., the metal pillars 118) electrically coupled to thefirst portion and extending from the respective standoff 140 to thesubstrate 102. The conductive traces 142 electrically couple respectivefirst pads 120 to respective metal pillars 118. The conductive traces142 extend along the respective standoff 140 to an opposing edge or side(e.g., under or rear side) of the standoff 140 opposite the second sidebonded to the interposer 104 such that a portion of the conductivetraces 142 is disposed between the opposing edge or side of the standoff140 and the metal pillar 118. The metal pillar 118 electrically couplesthe respective conductive traces 142 to the respective second pads 122(not illustrated in FIG. 1C) disposed on the first side of the substrate102 (e.g., via solder bump 126 attachment). Thus, each of the electricalconductors 114 (e.g., the electrically coupled combination of conductivetraces 142 and metal pillars 118) forms a respective electrical pathbetween one of a respective electrical contact 116 on the top side ofthe top-emitting VCSELs 108 and the substrate 102 thereunder.

In some examples, each of the electrical conductors 114 includes ananowire or nanotube. The nanowires extend between and electricallycouple the respective first pads 120 disposed on the second side of theinterposer 104 and the respective second pads 122 disposed on the firstside of the substrate 102. Each of the first pads 120 are electricallycoupled to one of the electrical contacts 116 of a respectivetop-emitting VCSEL 108. The nanowires can thus form electrical pathsbetween the respective electrical contacts 116 on top sides of thetop-emitting VCSELs 108 and the substrate 102 thereunder.

FIGS. 2A-2C illustrate examples of optical modules 200 (identifiedindividually as optical modules 200 a-200 c) and components thereofaccording to the present disclosure. Each of the optical modules 200a-200 c can include one or more of any of the components or features, inwhole or in part, of any of the features described herein with respectto each other as well as optical modules 100 a-100 c. For example, theoptical modules 200 can include a first interposer 204 similar oridentical to interposer 104 described above with respect to FIGS. 1A-1C.Further, the optical modules 200 each include a second interposer 252with opposing first and second sides disposed below the first interposer204 (e.g., the interposer 104). The second interposer 252 can be formedfrom Si, glass, or another suitable material. The optical modules 200include the second interposer 252 thereunder (e.g., in place of orinstead of the substrate 102 of the optical modules 100). The secondinterposer 252 can be disposed over or on a substrate (not illustratedin FIGS. 2A-2C) such as the substrate 102 described above with respectto FIGS. 1A-1C. In other examples, the second interposer 252 can bedisposed directly over or on an integrated circuit, chip, die, orprinted circuit board.

With reference to FIG. 2A, in some examples, the second interposer 252of optical module 200 a includes a plurality of trenches or cavities 250(identified individually as cavities 250 a-250 d). The cavities 250 canbe through holes in some examples. In other examples, the cavities 250can be blind holes. Top-emitting VCSELs 208 are each disposed in arespective cavity 250. The electrical conductors of the optical module200 a can include vias 254 (identified individually as vias 254 a-254d), for example, through substrate vias (TSVs). The second interposer252 can have a thickness or height of up to 400 micrometers. In someexamples, the second interposer 252 can have a thickness or height of200 micrometers. In other examples, the second interposer 252 can have athickness or height from 250 to 300 micrometers.

The vias 254 extend through the second interposer 252 to electricallycouple respective first pads 220 (identified individually as first pads220 a-220 d). The first pads 220 can be solder attachment pads disposedon the second side of the first interposer 204 to the second side of thesecond interposer 252. Opposing ends of the vias 254 can include solderbumps 226 to couple with the first pads 220 and a substrate (e.g., thesubstrate 102, integrated circuit (e.g., ASIC), chip, die, or printedcircuit board), respectively, under the second interposer 252.

As described above with respect to the optical module 100, each of thefirst pads 220 are electrically coupled to one of a pair of theelectrical contacts 216 on a top side of a respective top-emitting VCSEL208. The vias 254 thus form electrical paths between or from therespective electrical contacts 216 on top sides of the top-emittingVCSELs 208 through the second interposer 252 to the substrate (e.g., thesubstrate 102, integrated circuit (e.g., ASIC), chip, die, or printedcircuit board) thereunder. Additionally, as described above with respectto the optical module 100, the first pads 220 can be single padsextending along the interposer 204 and attached to both the electricalvias 254 and the top-emitting VCSELs 208. However, in other examples,the first pads 220 can include separate pads configured to be coupled tothe vias 254 and top-emitting VCSELS 208 respectively. The separate padscan be electrically coupled via conductive traces extending along orthrough the interposer 204.

The optical modules 200 can include other features as described abovewith respect to the optical modules 100 as described herein. Forexample, the optical modules 200 can also include additional pads 224mechanically coupled to the top-emitting VCSELs 208 via soldering (e.g.,solder bumps 226 and corresponding solder reflow techniques).Additionally, the optical modules 200 can include an optical underfilllayer 232 that is optically transparent and has an index of refractionmatching or substantially matching that of the first interposer 204 or athermally conductive underfill layer 234 under the optical underfilllayer 232. The optical modules 200 can also include one or more heatspreaders 236 a-236 d. Further, the optical modules 200 can also includea multiplexer/demultiplexer to multiplex or demultiplex optical signals,an optical fiber or other suitable waveguide to transmit or receive theoptical signals, as well as lenses 206 for collimating or focusing theoptical signals.

With reference to FIG. 2B, in some examples, the optical module 200 bincludes the second interposer 252 as described above with respect tooptical module 200 a and FIG. 2A. The optical module 200 b furtherincludes a plurality of first metal pillars 218 (identified individuallyas first metal pillars 218 a-218 d) and a plurality of second metalpillars 219 (identified individually as second metal pillars 219 a-219d). The first and second metal pillars 218 and 219 can be configuredidentically or similarly as metal pillar 118 of optical modules 100. Forexample, the first metal pillars 218 are fabricated on or otherwiseformed on the second side of the first interposer 204. Each of the firstmetal pillars 218 extends downward (e.g., towards the second interposer252 under the first interposer 204) from respective first pads 220disposed on the second side of the first interposer 204 between thefirst metal pillars 218 and the first interposer 204.

The second metal pillars 219 are fabricated on or otherwise formed onthe first side of the second interposer 252. Each of the second metalpillars 219 extends upward (e.g., towards the first interposer 204 abovethe second interposer 252) from respective second pads 222 (identifiedindividually as second pads 222 a-222 d) disposed on the first side ofthe second interposer 252 between the second metal pillars 219 and thesecond interposer 252. The first and second metal pillars 218 and 219are in “face-to-face” contact with each other. That is opposing ends ofthe first and second metal pillars 218 and 219 opposite of therespective interposers they are fabricated on are coupled together.Respective first and second metal pillars 218 and 219 are coupledtogether (e.g., via respective solder bumps 226) such that they are invertical or substantial vertical alignment with each other. Having twoor more metal pillars (e.g., pillars 218 and 219) extending fromrespective interposers and coupled together allows the metal pillars tobe shorter in height (e.g., 80 to 115 micrometers) relative to a singlemetal pillar (e.g., 175 to 225 micrometers) to provide sufficientclearance between the first and second interposers 204 and 254 for therespective top-emitting VCSELs 208. Metal pillars shorter in height maybe easier to fabricate.

As described above with respect to optical module 200 a, the interposer252 includes vias 254 extending through the second interposer 252. Thevias 254 can extend from the respective second pads 222 disposed on thefirst side of the second interposer 252 to the second side of the secondinterposer 252. While not illustrated in FIG. 2B, the ends of the vias254 at the second side of the second interposer 252 can includerespective solder bumps 226 to couple to a substrate (e.g., thesubstrate 102, integrated circuit (e.g., ASIC), chip, die, or printedcircuit board), respectively, under the second interposer 252. Further,as described above with respect to the first pads 120 of optical modules100, the first and second pads 220 and 222 of optical modules 200 can besingle pads or separate pads coupled together via electricallyconductive traces.

Each of the first pads 220 are electrically coupled to one of the pairof electrical contacts 216 on a top side of respective top-emittingVCSELs 208. The electrical conductors of the optical module 200 bincluding the electrically coupled first and second metal pillars 218and 219 and the vias 254 form electrical paths between or from therespective electrical contacts 216 on top sides of the top-emittingVCSELs 208 through the second interposer 252 to the substrate (e.g., thesubstrate 102, integrated circuit (e.g., ASIC), chip, die, or printedcircuit board) thereunder.

With reference to FIG. 2C, in some examples, the optical module 200 cincludes the second interposer 252 as described above with respect tooptical modules 200 a and 200 b as well as the plurality of first metalpillars 218 and second metal pillars 219. The optical module 200 cfurther includes a plurality of standoffs 240 (identified individuallyas standoffs 240 a-240 d) disposed on the second side of the firstinterposer 204 extending downward toward the second interposer 252. Thestandoffs 240 can be configured identically or similarly as standoffs140 described above bonded to or otherwise formed on the second side ofthe first interposer 204.

As illustrated, the first pillars 218 are fabricated on or otherwiseformed an under or rear side of respective standoffs 240. Each of thefirst metal pillars 218 extends downward (e.g., towards the secondinterposer 252 under the first interposer 204). The second metal pillars219 are fabricated on or otherwise formed on the first side of thesecond interposer 252. Each of the second metal pillars 219 extendsupward (e.g., towards the first interposer 204 above the secondinterposer 252) such that first and second metal pillars 218 and 219 arein “face-to-face” contact with each other.

Respective first and second metal pillars 218 and 219 can be coupledtogether (e.g., via respective solder bumps 226) such that they are invertical or substantial vertical alignment with each other as describedabove with respect to the metal pillars of optical module 200 b. Byincluding standoffs 240, the heights of the first and second metalpillars 218 and 219 can be further reduced or decreased relative tosingle metal pillars as well as the metal pillars of optical module 200b as the standoffs 240 provide additional clearance height between theinterposers 204 and 252. For example, with standoffs 240 having heightsof 125 micrometers, the metal pillars 218 and 219 can have heights of 25micrometers each such that sufficient clearance is provided fortop-emitting VCSELs of 150 micrometers in height to be disposed betweenthe interposers 204 and 252.

Each of the electrical conductors of the optical module 200 c includesconductive traces 242 (identified individually as conductive traces 242a-242 d). The conductive traces 242 can be configured identically orsimilarly as conductive traces 142 described above with respect tooptical modules 100 b and 100 c. For example, the conductive traces 242extend from the first pads 220 disposed on the second side of the firstinterposer 204 and along respective standoffs 240. The conductive traces242 electrically couple respective first pads 220 to respective metalpillars 218 extending from the standoffs 240. The conductive traces 242extend along the respective standoffs 240 to an opposing edge or side(e.g., under or rear side) of the standoff 240 opposite the second sidebonded to the interposer 240 such that a portion of the conductivetraces 242 is disposed between the opposing edge or side of the standoff240 and the metal pillar 218.

The metal pillars 218 and 219 electrically couple the respectiveconductive traces 242 to the respective second pads 222 disposed on thefirst side of the second interposer 252. Each of the first pads 220 areelectrically coupled to one of the pair of electrical contacts 216 on atop side of respective top-emitting VCSELs 208. The second interposer252 can include vias 254 as described herein. Thus, the electricalconductors of the optical module 200 c including the electricallycoupled first and second metal pillars 218 and 219, conductive traces242, and the vias 254 form electrical paths between or from therespective electrical contacts 216 on top sides of the top-emittingVCSELs 208, along the respective standoffs 240, through the secondinterposer 252, and to a substrate (e.g., the substrate 102, integratedcircuit (e.g., ASIC), chip, die, or printed circuit board) thereunder.

Referring to FIGS. 3A-3B, top section views of example die or packagelayouts of arrays of the optical modules as described herein (e.g.,optical modules 100 a-100 c, 200 a-200 c) are illustrated. As describedabove, top-emitting VCSELs 308 can be replaced with top-entryphotodetectors such that the optical transmitter or receivers can beconstructed as desired. The die layouts 300 a and 300 b can includesixteen top-emitting VCSELs 308 (e.g., in a 4×4 arrangement or layout)such that optical signals from each set or group of four top-emittingVCSELs 308 (identified individually as VCSELs 308 a-308 d) withdifferent wavelengths (e.g., λ₁, λ₂, λ₃, λ₄) are configured to bemultiplexed or otherwise combined to be transmitted over a respectiveoptical fiber 310 (identified individually as optical fibers 310 a-310d). In other examples, the die layouts 300 a and 300 b can include moreor less than sixteen top-emitting VCSELs 308. Each top-emitting VCSEL308 includes an active mesa 360 and light emitted from the top-emittingVCSELs 308 can be collimated by a lens 306 to be multiplexed asdescribed above.

Each top-emitting VCSEL 308 is flip-chipped or otherwise coupled to afirst interposer or substrate 304 (e.g., interposer 204 or substrate 104as described herein) via mechanical attachments M (e.g., pads 124 or 224and corresponding solder bumps). As described above, different channelsor wavelengths (e.g., λ₁, λ₂, λ₃, λ₄) of each set or group oftop-emitting VCSELs 308 can be combined and transmitted over arespective optical fiber 310. The VCSELs 308 configured to emit the samewavelength can be coupled to or otherwise formed on the same chip orVCSEL support substrate 305 (identified as VCSEL support substrates 305a-305 d) and then flip-chipped to the first interposer or substrate 304.For example, the VCSELs 308 a configured to emit wavelength λ₁ can beformed on the same VCSEL support substrate 305 a, the VCSELs 308 bconfigured to emit wavelength λ₂ can be formed on the same VCSEL supportsubstrate 305 b, the VCSELs 308 c configured to emit wavelength λ₃ canformed on the same VCSEL support substrate 305 c, and the VCSELs 308 dconfigured to emit wavelength λ₄ can be formed on the same VCSEL supportsubstrate 305 d.

Each top-emitting VCSEL 308 includes a pair of electrical contacts(e.g., identified as anode contact A and cathode contact C). Theelectrical contacts can be electrically coupled to a second interposeror substrate 352 (e.g., interposer 252 or substrate 102 as describedherein) under the first interposer or substrate 304 via electricalconductors 314. As described above with respect to the second interposeror substrate 252, the second interposer or substrate 352 can include aplurality of cavities or trenches spacing apart or between portions orsections of the second interposer or substrate 352 (identifiedindividually as second interposer or substrate sections 352 a-352 d).With reference to FIG. 3A, in some examples, the pairs of electricalcontacts of the VCSELs 308 can be disposed or positioned on a samelateral side of respective active mesas 360 of the VCSELs 308. Withreference to FIG. 3B, in other examples, the pairs of electricalcontacts of the VCSELs 308 can be disposed or positioned on opposinglateral sides of the respective active mesas 360 of the VCSELs 308.

The electrical conductors 314 can be configured according to any of theelectrical conductors described herein with respect to optical modules100 a-100 c, 200 a-200 c. For example, as illustrated in FIGS. 3A-3B,the electrical conductors 314 can include vias 354 extending through thesecond interposer or substrate 352 to form electrical paths between theelectrical contacts on a top-side of each top-emitting VCSEL 308 and thesecond side (e.g., under or rear side) of the second interposer orsubstrate 352 thereunder. As described above, the electrical paths caninclude corresponding pads and traces between the electrical contactsand vias. Further in some examples, the electrical conductors 314 canelectrically couple the electrical contacts to a substrate (e.g., thesubstrate 102, integrated circuit (e.g., ASIC), chip, die, or printedcircuit board) under the second interposer or substrate 352.

FIG. 4 illustrates an example optical module 400 and components thereofaccording to the present disclosure. The optical module 400 can includeone or more of any of the components or features, in whole or in part,of any of the features described herein with respect to any of opticalmodules 100 a-100 c or 200 a-200 c. For example, the optical module 400can include a first interposer 404 with opposing first and second sides(e.g., top and bottom or front and back surfaces, respectively) similaror identical to interposer 104 described above with respect to FIGS.1A-1C or first interposer 204 described above with respect to FIGS.2A-2C. For example, the first interposer 404 can be constructed of glassor other suitable materials with a relatively high-index of refraction(e.g., GaAs, GaP, GaN, InP). The first interposer 404 is constructed ofa material transparent to a range of wavelengths to be transmittedtherethrough. The first interposer 404 can be a passive glassinterposer. First and second sides of the first interposer 404 caninclude anti-reflective coatings. In some examples, the first interposer404 includes cavities 470 at the second side to accommodate respectivewire bonds 425 as described in more detail below.

Further, the optical module 400 includes a second interposer 452 withopposing first and second sides (e.g., top and bottom or front and backsurfaces, respectively) disposed below the first interposer 404 (e.g.,the interposer 104 or the first interposer 204). The second interposer452 can be formed from Si, SOI, or another suitable material orcombination of materials.

The optical module 400 includes the second interposer 452 thereunder(e.g., in place of or instead of the substrate 102 of the opticalmodules 100). However, the second interposer 400 can be disposeddirectly over or on a substrate (not illustrated in FIG. 4) such as thesubstrate 102 described above with respect to FIGS. 1A-1C. In otherexamples, the second interposer 452 can be disposed directly over or onan integrated circuit, chip, die, or printed circuit board. In thismanner, the optical module 400 is flip chip capable (e.g., the opticalmodule 400 can be flip-chip assembled to a substrate or other circuit,chip, board, or die thereunder). The second interposer 452 can have athickness or height of up to 400 micrometers. For examples, the secondinterposer 452 can have a thickness or height of 200 micrometers or 300micrometers. In other examples, the second interposer 452 can have athickness or height from 100 to 300 micrometers or from 300 to 350micrometers.

As illustrated, in some examples, the second interposer 452 of opticalmodule 400 includes a plurality of trenches or cavities 450 (identifiedindividually as cavities 450 a-450 d). The cavities 450 can be throughholes in some examples (e.g., extending completely through the secondinterposer 452 from the first side to the second side). In otherexamples, the cavities 450 can be blind holes (e.g., extending from thefirst side to an elevation spaced away from or above the second side).For example, the second interposer 452 can include an etch-stop layer453 such that blind hole cavities 450 are formed during processing(e.g., via DRIE) rather than through hole cavities. In such examples,the second interposer 452 can be formed from an SOI wafer with theetch-stop layer 453 being a BOX layer of the second interposer 452.

The cavities 450 can act as landing sites or locations within the secondinterposer 452 for respective top-emitting VCSELs 408 (identifiedindividually as top-emitting VCSELs 408 a-408 d) or top-entryphotodetectors to be positioned therein. For example, the top-emittingVCSELs 408 (e.g., or top-entry photodetectors) can each be disposed in arespective cavity 450 and coupled or otherwise bonded therein to thesecond interposer 452 (e.g., with thermally conductive glue or solder)such that the top-emitting VCSELs 408 (e.g., or top-entryphotodetectors) are affixed on or to the second interposer 452. Thetop-emitting VCSELs 408 (e.g., or top-entry photodetectors) can bepositioned within the respective cavities 450 via vision alignment.Alternatively, the top emitting VCSELs may be precisely positioned byplacing the VCSELs 408 against the sidewalls and bottom surface of thecavity 450.

Top surfaces of the top-emitting VCSELs 408 (e.g., or top-entryphotodetectors) can then be aligned with the first side or top surfaceof the second interposer 452 prior to coupling the top-emitting VCSELs408 (e.g., or top-entry photodetectors) thereto. For example, a diebonder equipped with alignment mechanisms (e.g., a collet with a touchprobe, optical proximity sensor) can be used to align the top-emittingVCSELs 408 (e.g., or top-entry photodetectors) with the secondinterposer 452 prior to coupling the top-emitting VCSELs 408 (e.g., ortop-entry photodetectors) thereto within the cavities 450. A verticaldistance between active regions of the VCSELs 408 and micro lenses onthe first interposer 404 (e.g., as described in more detail below) canbe properly maintained with such alignment and bonding techniques. Insome examples, each of the cavities 450 can include a thermallyconductive underfill 434 (e.g., similar to underfill 134 as describedabove) to improve heat flow to the second interposer 452 from the VCSELs408 or help maintain the VCSELs in position within the respectivecavities 450.

The optical module 400 includes electrical conductors 414 (identifiedindividually as electrical conductors 414 a-414 d). The electricalconductors 414 of the optical module 400 route electrical input/output(IO) from a top side of respective top-emitting VCSELs 408 (e.g., ortop-entry photodetectors) to a second side or bottom surface of thesecond interposer 452 (e.g., to a respective solder bump 426 or an arrayof solder bumps such that optical module 400 can be, for example,flip-chip assembled to a substrate or other circuit, chip, board, or dieas described above). The electrical conductors 414 include vias 454(identified individually as vias 454 a-454 d), for example, throughsubstrate vias (TSVs).

The vias 454 extend through the second interposer 452 to electricallycouple respective first pads 420 (identified individually as first pads420 a-420 d) on the first side of the second interposer 452 to thesolder bumps 426 at the second side of the second interposer 452. Forexample, the first pads 420 can be wirebond pads disposed on the firstside of the second interposer 452 electrically coupled with respectivesolder bumps 426 on the second side of the second interposer 452 by thevias 454. Ends of the vias 454 opposite the first pads 420 can includebump pads 421. Further, in some examples, the second side of the secondinterposer 452 can include a redistribution layer (e.g., RDL) such thatthe solder bumps 426 or bump pads 421 can be disposed away from oroffset (e.g., horizontally) from the first pads 420 or vias 454 to allowimproved accessibility as desired to the electrical 10 at the secondside of the interposer 452 (e.g., to flip-chip assemble the opticalmodule 400 to a substrate or other circuit, chip, board, or die via thesolder bumps 426).

Each of the first pads 420 are electrically coupled to one of a pair ofelectrical contacts 416 on a top side of a respective top-emitting VCSEL408 (e.g., or top-entry photodetectors as described herein). Forexample, each of the top-emitting VCSELs 408 can include respectivecomplementary or corresponding wire bond pads 423 in electricalcommunication with the electrical contacts 416. The corresponding orcomplementary wire bond pads 423 of the top-emitting VCSELs 408 can beelectrically coupled to respective first pads 420 (e.g., via respectivewire bonds 425). The vias 454 electrically couple the first pads 420 torespective solder bumps 426 through the second interposer 452 asdescribed above. The electrical conductors 414 thus form electricalpaths between or from the respective electrical contacts 416 on topsides of the top-emitting VCSELs 408 to the first side of the secondinterposer 452 and through the second interposer 452 to the second sideof the second interposer 452 (e.g., the solder bumps 426).

In some examples, the optical module 400 can include a hermetic sealingring 480 between the first interposer 404 and the second interposer 452configured to solder self-align the respective interposers together. Thehermetic sealing ring 480 can include complementary or correspondingring portions formed on the first and second interposers, respectively(e.g., identified as hermetic sealing ring portions 480 a and 480 b,respectively). The complementary ring portions can be soldered together.The hermetic sealing ring 480 can also protect the active regions of thetop-emitting VCSELs 408 or top-entry photodetectors or the wire bonds425 from exposure or damage from the environment or user contact in thefield. In other examples, the optical module 400 can include an opticalunderfill layer (e.g., layer 132 as described above with respect toother examples) between the interposers that is optically transparentand has an index of refraction matching or substantially matching thatof the first interposer 404 without a hermetic sealing ring. In suchexamples, the optical module can also include the thermally conductiveunderfill layer 434 under the optical underfill layer as describedherein.

The optical module 400 can include other features as described abovewith respect to the optical modules 100 a-100 c or 200 a-200 c asdescribed herein. For example, the optical module 400 can also include aplurality of lenses 406 (e.g., micro lenses) for collimating or focusingthe optical signals respectively transmitted or received through thefirst interposer 404. The plurality of lenses 406 (identifiedindividually as lenses 406 a-406 d) can be integrated on or otherwiseformed on the first side of the interposer 404. For example, the lenses406 may be fabricated via spin coating a polymer or other film on theinterposer 404, machined, or etched into the interposer 404. Whileillustrated as protruding out in a convex manner, in otherimplementations, the lenses 406 can be recessed within the interposer404.

The plurality of top-emitting vertical-cavity surface-emitting lasers(e.g., VCSELs 408) are configured to emit optical signals havingdifferent respective channels or wavelengths (e.g., λ₁, λ₂, λ₃, λ₄). Theoptical signals are configured to be combined and transmitted over asingle optical fiber 410 (e.g., via a multiplexer/demultiplexer 412), asdescribed herein. While illustrated as having four top-emitting VCSELs408 (identified individually as VCSELs 408 a, 408 b, 408 c, and 408 d),the optical module 400 can include more than four top-emitting VCSELs(e.g., eight, sixteen, thirty-two, sixty-four) and emit additionalwavelengths to be combined respectively (e.g., to increase overallbandwidth accordingly). A complementary or corresponding optical module(e.g., an optical receiving module) with top-entry photodetectors inplace of the top-emitting VCSELs 408 can be coupled to an opposing orsecond end of the optical fiber 410 to receive the optical signals fromthe optical module 400 coupled to the first end of the optical fiber 410and convert the optical signals to electrical signals for processing.

An example method of assembling or fabricating the optical module 400 isdescribed herein. The method can include one or more of any of thefollowing steps. The method can include forming pads (e.g., wire bondpads), a first complementary portion of a hermetic sealing ring, etchedlanding sites or cavities (e.g., via DRIE) for the top-entryphotodetectors or top-emitting VCSELs on the first side of the secondinterposer. The depth of the cavities may be different for each VCSELand photodetector. Through substrate vias can be formed through thesecond interposer to electrically connect or couple the pads on thefirst side of the second interposer to solder bumps on the second sideof the second interposer. The top-entry photodetectors or top-emittingVCSELs can be positioned within the landing sites via vision alignmentand affixed or otherwise bonded to the second interposer to the landingsites (e.g., with thermally conductive glue or solder).

As described above, a die bonder can be used to align the top surface ofthe top-entry photodetectors or top-emitting VCSELs with the first sideof the second interposer prior to bonding the top-entry photodetectorsor top-emitting VCSELs thereto. As such spacing or vertical distancebetween active regions of the top-entry photodetectors or top-emittingVCSELs and the first interposer can be selected and maintained. Wirebonds can be formed between wire bond pads of the top-entryphotodetectors or top-emitting VCSELs and the pads on the first side ofthe second interposer such that an electrical path is formed between thetop-entry photodetectors or top-emitting VCSELs and the second side ofthe second interposer. The wire bonds can be formed using conventionalhigh speed wire bonders in some examples.

The method can further include solder self-aligning a secondcomplementary portion of the hermetic sealing ring on the second side ofthe first interposer with the first complementary portion of thehermetic sealing ring to couple the first and second interposers andseal the wire bonds and top-entry photodetectors or top-emitting VCSELsfrom the environment or user interaction after the electrical conductorsare formed. As described above, the first interposer can be previouslyprovided with or processed to include a plurality of lenses or cavitiesfor the wire bonds prior to being coupled to the second interposer. Insome examples, when the optical module does not include the hermeticsealing ring, the method can include injecting the optical underfillbetween the respective interposers to couple or otherwise bond the firstand second interposers. After the first and second interposers arecoupled or otherwise bonded together, the method can include flip chipassembling the optical module to a substrate or other circuit, chip,board, or die, as described above.

In the foregoing description, numerous details are set forth to providean understanding of the subject disclosed herein. However,implementations may be practiced without some or all of these details.Other implementations may include additions, modifications, orvariations from the details discussed above. It is intended that theappended claims cover such modifications and variations. Thespecification and drawings are, accordingly, to be regarded asillustrative rather than restrictive. Additionally, in the interest ofclarity and to avoid unnecessarily obscuring the description, otherdetails describing well-known structures and systems often associatedwith optical modules (e.g., VCSEL contact pads, traces between pads,driver circuitry), have not been set forth herein in the description ofthe various examples of the present disclosure.

It will be recognized that the terms “comprising,” “including,” and“having,” as used herein, are specifically intended to be read asopen-ended terms of art. The term “or,” in reference to a list of two ormore items, covers all of the following interpretations of the word: anyof the items in the list, all of the items in the list, and anycombination of the items in the list. As used herein, the terms“connected,” “coupled,” or any variant thereof means any connection orcoupling, either direct or indirect (e.g., having additional interveningcomponents or elements), between two or more elements, nodes, orcomponents; the coupling or connection between the elements can bephysical, mechanical, logical, optical, electrical, or a combinationthereof.

In the Figures, identical reference numbers identify identical, or atleast generally similar, elements. To facilitate the discussion of anyparticular element, the most significant digit or digits of anyreference number refers to the Figure in which that element is firstintroduced. For example, element 110 is first introduced and discussedwith reference to FIG. 1.

The invention claimed is:
 1. An optical transmitter module comprising: afirst interposer having opposing first and second sides; a secondinterposer having opposing first and second sides; a plurality oftop-emitting vertical-cavity surface-emitting lasers (VCSELs) coupled tothe second interposer, the top-emitting VCSELs configured to emitoptical signals having different wavelengths through the firstinterposer, the optical signals configured to be combined andtransmitted over a single optical fiber; and a plurality of electricalconductors forming electrical paths between electrical contacts of thetop-emitting VCSELs and the second side of the second interposer,wherein the second interposer includes a plurality of cavities, whereineach of the top-emitting VCSELs is disposed in a respective cavity, andwherein each of the electrical conductors extends through the secondinterposer to electrically couple respective first pads disposed on thefirst side of the second interposer to a respective solder bump on thesecond side of the second interposer, each of the first padselectrically coupled to one of the electrical contacts of a respectivetop-emitting VCSEL.
 2. The optical module of claim 1, wherein theelectrical contacts of each top-emitting VCSEL include an anode contactand a cathode contact disposed on a top side of the top-emitting VCSELsuch that respective electrical paths extend from the second interposerto the top-side of the top-emitting VCSEL.
 3. The optical module ofclaim 1 wherein each of the cavities are blind holes formed through thesecond interposer.
 4. The optical module of claim 3, wherein each of thecavities include a thermally conductive underfill layer therein.
 5. Theoptical module of claim 1, wherein the electrical conductors comprisevias extending through the second interposer electrically couplingrespective first pads disposed on the first side of the secondinterposer to respective solder bumps on the second side of the secondinterposer.
 6. The optical module of claim 1, wherein each of thetop-emitting VCSELs includes a wire bond pad, each of the wire bond padselectrically coupled to respective electrical contacts of thetop-emitting VCSELs, and wherein each of the wire bond pads areelectrically coupled to respective first pads disposed on the first sideof the second interposer via respective wire bonds.
 7. The opticalmodule of claim 6, wherein the second side of the first interposerincludes a plurality of cavities configured to allow a respective wirebond to extend therein when the first interposer is bonded to the secondinterposer.
 8. The optical module of claim 1, wherein the firstinterposer is a glass interposer.
 9. The optical module of claim 8,further comprising a plurality of lenses on the first side of the firstinterposer.
 10. The optical module of claim 1, further comprising ahermetic sealing ring extending from the second side of the firstinterposer to the first side of the second interposer configured tosolder self-align the first interposer to the second interposer.
 11. Anoptical receiving module comprising: a first interposer having opposingfirst and second sides; a second interposer having opposing first andsecond sides; a plurality of top-entry photodetectors coupled to thesecond interposer, the top-entry photodetectors configured to receiveoptical signals having different wavelengths through the firstinterposer, the optical signals configured to be received from a singleoptical fiber and demultiplexed such that each of the top-entryphotodetectors receives an optical signal with a different respectivewavelength; and a plurality of electrical conductors forming electricalpaths between electrical contacts of the top-entry photodetectors andthe second side of the second interposer, wherein the second interposerincludes a plurality of cavities, wherein each of the top-entryphotodetectors is disposed in a respective cavity, and wherein each ofthe electrical conductors extends through the second interposer toelectrically couple respective first pads disposed on the first side ofthe second interposer to a respective solder bump on the second side ofthe second interposer, each of the first pads electrically coupled toone of the electrical contacts of a respective top-entry photodetector.12. The optical module of claim 11, wherein each of the cavities areblind holes formed through the second interposer.
 13. The optical moduleof claim 12, wherein each of the cavities include a thermally conductiveunderfill layer therein.
 14. The optical module of claim 11, wherein theelectrical conductors comprise vias extending through the secondinterposer electrically coupling respective first pads disposed on thefirst side of the second interposer to respective solder bumps on thesecond side of the second interposer.
 15. The optical module of claim11, wherein each of the top-entry photodetectors includes a wire bondpad, each of the wire bond pads electrically coupled to respectiveelectrical contacts of the top-entry photodetectors, and wherein each ofthe wire bond pads are electrically coupled to respective first padsdisposed on the first side of the second interposer via respective wirebonds.
 16. The optical module of claim 11, further comprising a hermeticsealing ring extending from the second side of the first interposer tothe first side of the second interposer configured to solder self-alignthe first interposer to the second interposer.
 17. An optical systemcomprising: an optical transmitter comprising: a first interposer havingopposing first and second sides; a second interposer having opposingfirst and second sides; a plurality of top-emitting vertical-cavitysurface-emitting lasers (VCSELs) coupled to the second interposer, thetop-emitting VCSELs configured to emit optical signals having differentwavelengths through the first interposer, the optical signals configuredto be combined and transmitted over a single optical fiber; a pluralityof electrical conductors forming electrical paths between electricalcontacts of the top-emitting VCSELs and the second side of the secondinterposer; and a multiplexer; an optical receiver comprising: a firstinterposer having opposing first and second sides; a second interposerhaving opposing first and second sides; a plurality of top-entryphotodetectors coupled to the second interposer, the top-entryphotodetectors configured to receive optical signals having differentwavelengths through the first interposer, the optical signals configuredto be received from a single optical fiber and demultiplexed such thateach of the top-entry photodetectors receives an optical signal with adifferent respective wavelength; a plurality of electrical conductorsforming electrical paths between electrical contacts of the top-entryphotodetectors and the second side of the second interposer; and ademultiplexer; and an optical fiber having opposing ends coupled to theoptical transmitter and optical receiver respectively, the opticalsignals configured to be combined via the multiplexer and transmitted tothe optical receiver via the optical fiber, the demultiplexer configuredto separate the optical signals transmitted via the optical fiber intodifferent wavelengths to be received by respective top-entryphotodetectors of the optical receiver, wherein the second interposer ofthe optical transmitter includes a plurality of cavities, wherein eachof the top-emitting VCSELs is disposed in a respective cavity, andwherein each of the electrical conductors extends through the secondinterposer to electrically couple respective first pads disposed on thefirst side of the second interposer to a respective solder bump on thesecond side of the second interposer, each of the first padselectrically coupled to one of the electrical contacts of a respectivetop-emitting VCSEL.